In MPEG-2 video, a B-picture can be encoded by one picture displayed before, called the forward reference picture and one picture displayed after, called the backward reference picture. Encoded B-pictures are data dependent on these reference pictures. The reference pictures are decoded prior to the B-picture. One of the reference pictures, however is displayed after the B-picture.
As a result, the decoding order and the displaying order can be different. The decoder system receives the pictures in the decoding order. After decoding the pictures, the decoder system reorders the pictures into the display order. A display engine displays the reordered pictures.
A multimedia program can include a video, multiple audio channels, and data channels. The video, audio channels, and data channels are synchronized to make the program intelligible. Time stamps are used to achieve this synchronization.
As noted above, the video pictures have a decoding order and a display time. Decoding time stamps (DTS) indicate the decoding time. Presentation time stamps (PTS) indicate the display order. The video encoder encodes the DTS and PTS into the video program. The DTS and PTS are 33-bit numbers that are driven by a 27 MHz clock.
The DTS and PTS add overhead to the transmitted stream. Encoders encode the time stamps on a periodic basis. MPEG allows as much as 700 ms between time stamps. For a display rate of 30 interlaced pictures/sec (60 fields/sec.), there can be as many as 41 consecutive fields without time stamps.
A decoder system can interpolate the DTS and PTS values of the pictures without time stamps. The decoding and presentation times are spaced evenly apart. The decoder system can interpolate the decode time because the pictures are provided in decode order. The decoder system interpolates the presentation time by evaluating the type of pictures. However, interpolating the PTS values involves decoding numerous parameters and pictures. This can be unfeasible where a large number of consecutive pictures are without time stamps. This is particular complex in cases where there are a large number of consecutive B-pictures.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of ordinary skill in the art through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.